There is one number heard more than any other on the podiums of the United Nations climate summit in Glasgow, Scotland: 1.5 degrees Celsius.

This is the global goal on climate change that world leaders have agreed to achieve. By limiting global warming to 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, by 2100, the hope is to avoid severe climate disruption that could exacerbate hunger, conflict and drought in the world. world.

The 1.5 degree target has long been championed by developing countries, where millions of people are among the most vulnerable to climate change. During the Paris climate negotiations in 2015, they pushed industrialized countries to improve the 2 degrees Celsius target set at the time, because the richest countries are responsible for most of the greenhouse gas emissions. greenhouse since the industrial revolution.

In the ongoing climate negotiations, countries are touting new commitments to reduce their heat-trapping emissions by switching to clean energy and reducing deforestation. India commits, for the first time, to be carbon neutral by 2070. More than 100 countries, including the United States, have joined a global pact to reduce methane, a powerful greenhouse gas .

Yet, added together, recent commitments do not go far enough. Even with more ambitious emission cuts from some countries, warming is still on track to more than 2 degrees Celsius (or 3.6 Fahrenheit) by the end of the century. The Earth is already 1.1 degrees Celsius warmer than it was 150 years ago.

While a difference of half a degree Celsius in the increase in temperature may seem inconsequential, the difference for life on Earth could be huge. Here’s what scientists expect if average global temperatures rise above 1.5 degrees Celsius by 2100.

Coral reefs face almost complete disappearance

Off the coast of Australia, the Great Barrier Reef is known to be large enough to be seen from space. That’s the size of Germany – a biodiversity hotspot that was once considered too big to fail. But in recent decades, marine biologists like Ove Hoegh-Guldberg of the University of Queensland have observed its rapid decline.

“My career has been going from a time when it was wonderful and abundant to now, looking through the barrel,” Hoegh-Guldberg said. “We’re looking at the cannon of something really horrible.”

The oceans heat up as the atmosphere warms, as they absorb much of the excess heat due to climate change. Repeated marine heat waves over the past five years have turned much of the Great Barrier Reef to a ghostly white color. When temperatures rise, corals expel the microscopic algae they contain, thus losing their food source. Sometimes corals can recover, but more and more they die.

“About 50% of shallow water corals have been killed literally within months, in some cases within weeks,” says Hoegh-Guldberg. “If you extend this into the future, we’ll get to a point where the damage is beyond the ability of the corals to bounce back.”

Marine heat waves have already doubled since 1980 and are expected to intensify as temperatures rise. At 1.5 degrees Celsius, it is likely that 70-90% of the world’s coral reefs are dying. At 2 degrees Celsius of warming, 99% is lost.

“If we delay for another year or two, we’re really going down a path where there’s no going back,” Hoegh-Guldberg said. “We must act and we must act decisively, without asking questions and solving this problem.”

“Unheard of” storms become more frequent

Water has taken a heavy toll in 2021 around the world. In September, the remnants of Hurricane Ida swept through the northeast, killing more than 50 people in New Jersey, New York and other states. Many drowned in cars and basement apartments, submerged in swift waters. In August, two dozen people died when heavy rains caused flash floods in Tennessee.

Scientists warn that a warmer atmosphere is a more humid atmosphere. Warmer air can hold more water, which helps produce more intense precipitation and stronger storms. With warmer ocean temperatures in the Gulf of Mexico, hurricanes are intensifying at a faster rate.

As the climate warms, storms once considered extremely rare are expected to become more frequent. And the odds don’t increase a bit.

“When we think about probabilities, it’s not just that twice as much warming gives you twice as much of a chance,” says Gabriel Vecchi, professor of geosciences at Princeton University. “If we move towards 3 degree warming, more and more of the unknown or unknown will become relatively common.”

Extreme heat waves follow the same pattern, where a small change in average temperature leads to a large increase in the number of extreme events. Whether it’s heat or flooding, infrastructure like buildings, roads and storm drains have been designed to cope with the climate of the past. They might not survive the climate of the future.

Melting ice results in flooded cities

For coastal cities around the world, the future of millions of people depends in large part on the vast volumes of ice present over Greenland and Antarctica.

Already in the past 16 years, Antarctica and Greenland have lost enough ice to fill Lake Michigan, according to NASA. This melting ice is raising the level of the oceans, as well as the melting of glaciers and the expansion of the ocean water itself, which occurs as it gets warmer. The increase is also accelerating.

Sea level could rise an additional 1 to 3 feet by 2100, according to the latest report from the Intergovernmental Panel on Climate Change, the UN’s climate research group. The scenario could be much worse if the Antarctic ice caps disintegrate at a faster rate than scientists predict. Limiting temperature increases to 1.5 degrees Celsius could halve the sea level rise that occurs by the end of the century, compared to what is expected.

More than 4 million people in the United States are at risk along the coasts, where rising sea levels would cause larger storm surges and higher high tides. As the ocean rises, many of the world’s small island nations are at risk of becoming uninhabitable.

Yet even at 1.5 degrees Celsius of warming, scientists warn that storms, heat waves and droughts will be more extreme. And they warn that 1.5 degrees Celsius of warming is not a tipping point. For every tenth of a degree, the planet gets hotter, the impacts get worse. But on the other hand, every tenth of a degree that is avoided can be crucial in limiting the extent of future damage.

“There are possible ways forward,” said Vecchi. “It’s not an inevitable future. There are reasons to hope and despair, I think, it’s counterproductive.”

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